If you're really stuck on getting the B&M brand lights, then my advice will be of no help. If you're not particular to brand, then read on.

I just installed a Supernova E3 3-LED tail light yesterday afternoon, and it works incredibly well. First, let me say that since I use exclusively LED lighting, I leave my lights on 100% of the time so there are no concerns with 'motion sensing' or 'light sensing' mechanisms. Always on, no worries.
I was at my LBS and saw the hulking enormity of the 4D Toplight and immediately dropped it from my list. My commuter is also my brevet bike, it has no rack, and I wasn't about to attempt rigging up a mount for that monstrosity. The E3, OTOH, is about the size of 2 sugar cubes on a mounting plate which is just wide enough to span the mount holes on a standard rear rack. It weighs about as much as a pair of AAA batteries if you include the wiring and all the hardware.

I used a pair of p-clips to mount the light above my brake bridge on the seat stay. This angles the light back and up, and I've aimed it slightly out toward the traffic lane. The E3 tail light is 180-degree viewable, so even with the angled placement it is very visible. Additionally, the thing is just insanely bright. On the unlighted MUP I looked down and back at it while riding and saw that in addition to throwing a nice red wash over my rear fender, it illuminates the underside of my seat bag and my reflective triangle hanging from the bag-loops on my saddle.